This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
For the time being, Licensed Assisted Access Long-Term Evolution (LAA-LTE) and LTE over Unlicensed carrier in Stand Alone mode (LTE-USA) are under discussion.
For LAA LTE, a network shall be operated in a carrier aggregation (CA) mode, a primary component carrier (CC) shall be a licensed CC, and control functionality is mainly via the primary CC (PCC). The unlicensed CC can only be the secondary CCs (SCCs). Configuration and activation/de-activation of the unlicensed CC follow a typical procedure for CA. Network discovery and mobility management are based on measurements based on the PCC. FIG. 1 illustratively shows an example of CA of LAA-LTE. As shown in FIG. 1, there is one licensed CC in a downlink (DL) and one licensed CC in an uplink (UL) as the PCC for DL and the PCC for UL, respectively (of course, more than one licensed CC in DL and more than one licensed CC in UL as the PCC for DL and the PCC for UL respectively are possible); meanwhile, there are multiple unlicensed CCs either in DL or UL or in both DL and UL.
For LTE-USA, there is no licensed CC to be used as a PCC. That is, the control functionalities that are managed based on the licensed CC in LAA-LTE shall be realized via unlicensed CCs in LTE-USA. Due to channel availability of the unlicensed CC is opportunistic, control signals and related procedures shall be modified in relation to restriction of channel availability. FIG. 2 illustratively shows an example of CA of LTE-USA. As shown in FIG. 2, there is one unlicensed CC in DL and one unlicensed CC in UL as the PCC for DL and the PCC for UL, respectively (of course, more than one unlicensed CC in DL and more than one unlicensed CC in UL as the PCC for DL and the PCC for UL respectively are possible); meanwhile, there are multiple unlicensed CCs either in DL or UL or in both DL and UL.
One issue of LTE-USA is that some basic reference signals (referred to as Discovery Reference Signals, DRS) used for network discovery shall be adaptively transmitted based on carrier sensing results, instead of transmitting at predefined time. The DRS may comprise Primary Synchronization Signal (PSS), Secondary Synchronization Signal (SSS), Cell-specific Reference Signal (CRS), Channel State Information-Reference Signal (CSI-RS), etc. The DRS is further allowed to move within a DRS Measurement Time Configuration (DMTC) window. In such condition, a UE shall monitor the DRS by tentatively demodulating the DRS in multiple possible places within the DMTC window until the DRS is identified. A typical DMTC window size is 6 subframes while a DMTC period may be 40 or 80 ms. FIG. 3 illustratively shows a DRS monitoring in LTE-USA.
Another issue of LTE-USA is that the DRS will not be transmitted as frequently as licensed CC, since the unlicensed CC shall be shared between coexisting networks and a frequent transmission of DRS will dramatically reduce resource efficiency. Besides, MIB (Master Information Block) system information will be transmitted along with DRS transmission for LTE-USA.
To receive paging messages from a radio access network, e.g. E-UTRAN, UEs in idle mode monitor a Physical Downlink Control Channel (PDCCH) for a Radio Network Temporary Identity (RNTI) value used to indicate paging, i.e., Paging-RNTI (P-RNTI). The UE only needs to monitor the PDCCH channel at certain UE-specific occasions. At other times, the UE may apply Discontinuous Reception (DRX), meaning that the UE can switch off its receiver to preserve battery power. The E-UTRAN configures which of radio frames and subframes are used for paging. Each cell broadcasts a default paging cycle, which is also referred to as a cell-specific default paging cycle. In addition, upper layers may use a dedicated signaling to configure a UE-specific paging cycle. If both of the cell-specific default paging cycle and the UE-specific paging cycle are configured, the UE applies the lowest value as a paging cycle of the UE. The UE calculates the radio frame (i.e., the Paging Frame (PF)) and the subframe within that PF (i.e., the Paging Occasion (PO)) which E-UTRAN applies to page the UE according to Formula (1) as follows:SFN (i.e., System Frame Number) mod T=(T/N)×(UE_ID mod N)  (1)                i_s=└UE_ID/N┘ mod Ns        T=UE DRX cycle (i.e., paging cycle)=min(Tc, Tue)        N=min(T, nB)        Ns=max(1, nB/T)        where:                    Tc is a cell-specific default paging cycle {32, 64, 128, 256} radio frames;            Tue is a UE-specific paging cycle {32, 64, 128, 256} radio frames;            N is the number of paging frames within a paging cycle T of the UE;            UE_ID is International Mobile Subscriber ID (IMSI) mod 1024, with IMSI being a decimal rather than a binary number;            i_s is an index pointing to a pre-defined table defining a corresponding subframe,            nB is the number of ‘paging subframes’ per paging cycle (across all UEs in the cell),            Ns is the number of ‘paging subframes’ in a radio frame that is used for paging.                        
Table 1 below lists a number of examples to illustrate calculation of the paging radio frames (i.e., PF) and subframes (i.e., PO).
TABLE 1CaseUE_IDTeTueTnBNNsPFi_sPOA147256256256646417609B147256128128323217609C14725612812825612821914
In cases A and B in Table 1, one out of every four radio frames is used for paging, using one subframe in each of those radio frames. For case B, there are 32 paging frames within the paging cycle of the UE, across which the UEs are distributed based on the UE_ID. In case C, two subframes in each radio frame are used for paging, i.e. Ns=2. In this case, there are 128 paging frames within the paging cycle of the UE and the UEs are also distributed across the two subframes within the paging frame. The LTE specifications include a table that indicates the subframe applicable for each combination of Ns and i_s, which is the index that follows from Formula (1). FIG. 4 illustrates cases B and C of PF and PO. All the shaded subframes can be used for paging; and the darker ones are applicable for the UE with the indicated identity.
In traditional LTE systems operating in a licensed band, i.e., LAA-LTE, idle UEs configured with DRX will wake up in every paging cycle. A main purpose is to see if there is paging message for the UE itself. If there is no paging message for the UE itself, a time/frequency synchronization tracking is performed with DRS (e.g. including CRS) transmission in each DL subframe. However, in standalone LTE in an unlicensed band, i.e., LTE-USA, there will be no always DRS transmission for time/frequency synchronization tracking, if paging transmission fails based on a carrier sensing scheme such as Listening Before Talk (LBT) or there is no paging message. In this case, UEs couldn't make synchronization tracking to adjust e.g. a clock drift in LTE-USA, if the paging message is transmitted using the same mechanism as in LAA-LTE.
Hence, there may be some disadvantages in terms of power consumption and channel availability if conventional paging methods are used for LTE-USA. In particular, there may be some aspects of disadvantages as described below with conventional paging methods being used for LTE-USA:                As a first aspect, a UE has to monitor both a DRS and a paging message individually if the DRS and the paging messages are sent via separate time windows. The power consumption at the UE side can increase largely.        As a second aspect, channel utility may be wasteful just for the paging message transmission. The paging message transmission will block a neighboring node to use the channel. If there are multiple neighboring nodes, generated load of paging messages could be considerable.        
Therefore, paging related technical solutions are desired which can solve at least the above problems.